This invention relates to an internal combustion engine in which the air-to-fuel ratio of the air-fuel mixture supplied into the combustion chambers of the engine is controlled utilizing feedback techniques in accordance with the composition of the exhaust gases discharged from the combustion chambers of the engine.
As is well known, it is now required from standpoints of exhaust gas control and fuel economy to accurately control the air-to-fuel ratio of the air-fuel mixture supplied into the combustion chambers of an internal combustion engine at a required value. Especially when the exhaust system of the internal combustion engine is equipped with a three-way catalytic converter capable of reducing nitrogen oxides as well as oxidizing hydrocarbons and carbon monoxide, the three-way catalytic converter requires to be supplied with an exhaust gases produced by combustion of the air-fuel mixture having approximately stoichiometric air-to-fuel ratio in the combustion chambers in order to allow the converter to function effectively and sufficiently. However, usual carburetors can not control so accurately the air-to-fuel ratio of the mixture supplied therefrom due to their constructions and characteristics.
In order to satisfy the above requirements, it has been proposed that the air-to-fuel ratio of the mixture supplied into the combustion chambers is controlled at the stoichiometric air-to-fuel ratio utilizing feedback techniques wherein the fuel amount supplied into the combustion chambers is directly or indirectly regulated in response to the composition of the exhaust gases which composition are detected by an exhaust gas sensor located in an exhaust passage communicated downstream of the combustion chambers of the engine. This method has been realized depending on the fact that the composition of the exhaust gases are in close relationship with the air-to-fuel ratio of the mixture supplied into the combustion chambers of the engine. In a system for performing the above method, the regulation of the fuel amount supplied to the combustion chambers is accomplished by controlling the air amount inducted through an additional air passage which communicates a main well of the carburetor with the atmosphere. The additional air passage is formed in addition to a main air bleed which is arranged to induct atmospheric air into the main well. The control of the air amount inducted through the additional air passage is carried out by valve means which is arranged to close or open in on-and-off manner the additional air passage, and to increase or decrease the air amount inducted therethrough than a predetermined amount by decreasing or increasing the time rate for closing the additional air passage than a predetermined rate.
However, this system employing such type of the inducted air amount control mechanism has encountered the following difficulty: when the throttle valve of the carburetor is opened, the additional air passage is supplied with a strong vacuum which is applied through a main discharge nozzle and the main well. However, when the engine is decelerated and the throttle valve is abruptly almost closed, the main discharge nozzle is subjected to a weak vacuum near the atmospheric pressure, but the strong vacuum is still remains in the additional air passage when the passage is closed by the valve means and accordingly the fuel in the main well is sucked and stayed in the additional air passage. The sucked and stayed fuel is thereafter discharged through the main discharge nozzle when the passage is opened and therefore invites disturbance of the control of the fuel amount discharged through the main discharge nozzle. This fuel control disturbance results in so-called car knock or undesirable frequent change of the vehicle speed.